Security experts say it's the biggest worm attack in years, call it "amazing" and report that it infected nearly 9 million PCs in just two weeks.

Downadup is downright nasty. And that's even before it does much more than just spread.

But as analysts argue about how the compromised computers will be used -- to build a massive botnet, perhaps -- or how much information hackers will steal from infected machines, users like you have a more immediate concern: "How do I keep my PC from joining the ranks of the hacked?"

That's a simple question. Unfortunately, because of this worm's flexibility, the answers aren't.

What's the worm again? Thanks to the lack of an industry-wide labeling system, the worm goes by more than one name. Some companies dub it "Downadup," others call it "Conficker."

No matter the name, it's the same threat.

When did Downadup first appear? Security companies warned of the worm in late November 2008; Symantec Corp. was one of the first to sound the alarm when it raised its ThreatCon security alert level on Nov. 21. Within a week, Microsoft Corp. had added its voice to the chorus as it acknowledged a significant uptick in attacks.

How does it spread? One of Downadup's most intriguing aspects, say security researchers, is its multipronged attack strategy: It can spread three different ways.

The one that's gotten the most attention exploits a vulnerability in Windows that Microsoft patched nearly four months ago. The bug, which is in a file-sharing service that's included in all versions of the operating system, can be exploited remotely just by sending a malformed data packet to an unpatched PC.

But the worm can also spread by brute-force password attacks, and by copying itself to any removable USB-based devices such as flash drives and cameras. More on those two in a moment.

What machines are most vulnerable to Downadup attack? According to Microsoft, unpatched Windows 2000, Windows XP and Windows Server 2003 machines are at the greatest risk to exploits of the bug patched in October. That gibes with reports from security companies, which have highlighted the danger to PCs running Windows XP Service Pack 2 and XP SP3. Not coincidentally, those versions account for the bulk of Windows' market share.

Unpatched Windows Vista and Server 2008 systems, meanwhile, are less likely to fall victim to attack, since hackers must have authenticated access to the computer, or in other words, know the log-in username and password.

Any Windows-powered machines, however, can be compromised by the worm's password and USB attack strategies.

Microsoft offered the fix as a security patch to users of the Windows 7 "pre-beta," the version it gave developers in late October and early November. It then integrated the patch into Windows 7 before it launched the public beta on Jan. 10.

OK, so how do I protect my PC? Because this thing is a triple threat, you'll need to take more than one defensive measure.

First of all, if you haven't already done so, apply the October fix that Microsoft tagged as MS08-067. If you have Windows Update set to automatically download and install patches, you should be protected, but it never hurts to double-check. You can verify that the patch has been installed by bringing up Windows Update, then clicking "Review your update history" and looking for a security update labeled as "KB958644."

If you are only now installing the patch, you might want to take Microsoft's advice and also download and install the January edition of its free Malicious Software Removal Tool (MSRT), which was updated last week so that it can detect, and then delete, Downadup infections.

What's this about password attacks? Although most of the news about Downadup's spread has focused on its exploitation of a patched bug in Windows, the worm also propagates by trying to guess other machines' administrative passwords.

Once the worm penetrates a corporate network -- perhaps by infecting a single unpatched machine, say a laptop, that is later connected to that network -- it tries to break into other PCs, including those that have been patched with the October emergency fix.

"One of the ways in which the Conficker worm (also known as Confick or Downadup) uses to spread is to try and batter its way into ADMIN$ shares using a long list of different passwords," said Graham Cluley, a senior technology consultant at Sophos, in an entry to a company blog last Friday. Cluley included the list of passwords that Downadup tries, which range from the ubiquitous password and the moronic secure to the slightly-more-clever letmein and nimda, or admin spelled backward.

Cluley urged users to steer clear of what he called "poorly-chosen passwords," while other security companies recommended that users not only pick stronger passwords but change them periodically as well.

Obviously, if you're using a password that's on the Downadup list, you should change it immediately.

And the worm can spread from flash drives, too? Yes.

From the moment Downadup infects a PC, it copies a file, named "autorun.inf" to the root of any USB storage devices, typically flash drives, that are connected to the compromised computer. That file name takes advantage of Windows' Autorun and Autoplay features to copy the worm to any machine that a flash drive, camera or other USB device is plugged into. Downadup will infect that PC when the drive or device is connected, or when the user double-clicks the device's icon within Windows Explorer or from the desktop.

Security experts have recommended that users disable both Autorun and Autoplay in Windows.

A December blog post by Symantec researcher Ben Nahorney spells out how to disable Autoplay, while a separate post on the Hackology blog outlines how to turn off Autorun by editing the registry.

What are the signs that my PC has been hit? Microsoft's advisory about Downadup lists several symptoms of infection, including these:

Account lockout policies are being tripped (because your password's been hijacked, and changed, by the attacker).

Automatic Updates are disabled (because Downadup tries to keep the PC unpatched by turning off Windows Update's automatic update, as well as Background Intelligent Transfer Service (BITS), the Windows component used by Windows Update to actually deliver the updates).

Various security-related Web sites cannot be accessed (because Downadup blocks access to a whole host of security companies' sites in an effort to prevent antivirus software from being updated, which could result in the worm's detection and eradication).

If your PC is exhibiting any of these symptoms -- or the others that Microsoft spells out here -- the company recommends that you immediately use the MSRT to clean the machine.